1. Field of the Invention
This invention relates to liquid crystal device alignment. Such devices typically comprise a thin layer of a liquid crystal material contained cell walls. Optically transparent electrode structures on the walls allow an electric field to be applied across the layer causing a re-ordering of the liquid crystal molecules to an ON state. On removing the electric field the molecules relax back to their OFF state.
2. Discussion of Prior Art
There are three known types of liquid crystal material, nematic, cholesteric, and smectic each having a different molecular ordering.
The present invention particularly concerns devices using nematic, long pitch cholesteric or ferroelectric materials, and a surface alignment treatment to the cell walls. This surface alignment aligns liquid crystal molecules in contact with the wall in an alignment direction. By arranging these alignment directions orthogonal the liquid crystal is forced to adopt a twisted structure in its voltage OFF state. This device is known as a twisted nematic device. Addition of a small amount of cholesteric material to the nematic material imparts a preferred twist direction to ensure a uniform twist in the device. Also devices can be made with angles of twist greater than 90.degree.; eg the super twisted nematic device, or 270.degree. twisted nematic device described in U.S. Pat No. 4,596,446. Another requirement for the alignment treatment is that it should also impart a surface tilt to liquid crystal molecules at the cell wall. Such a surface tilt is necessary in some devices to ensure a uniform display as described eg in GB-1,472,247, and 1,478,592.
One method of providing alignment is termed a rubbing process where a cell wall, with or without a polymer layer, is unidirectionally rubbed by a soft cloth. Liquid crystal molecules align along the rubbing direction, usually with a surface tilt of about 2.degree. or more depending upon the polymer layer.
Another alignment technique is known as oblique evaporation of eg SiO, which can produce surface tilts of zero, or high tilt eg 30.degree. depending upon evaporation direction angle. Such a technique is cumbersome for large scale manufacture; but a more important problem is that it is difficult to provide a uniform alignment direction and surface tilt over large areas of cell walls.
Short pitch cholesteric materials forming thermochromic displays have been aligned by grating structures embossed into plastic cell walls; this is described in GB 2143323 (McDonnell, 1983). A previous use of gratings to obtain pretilted alignment has utilised a blazed grating crossed with a sinusoidal grating (U.S. Pat. No. 4,521,080, Funada et al, 1985, Lee et al SID 93 Digest p. 957, Japan-A-60 060 624, and GB 2 286 894).
The liquid crystal director then runs along the sinusoidal grooves and hence over the blazed grooves which leads to surface pretilt. Pretilted alignment is essential for liquid crystal devices to avoid reverse tilt disclinations which can lead to poor device quality. One disadvantage of this prior art is that it only provides a limited range of pretilts.